Docs: document E049, E050, E051

Author: strager

docs/errors/E049.md

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+# E049: binary number literal has no digits
+
+Decimal number literals start with `0b` and require at least one digit. It is an
+error to write `0b` with no following digits:
+
+    let mask = 0b
+
+To fix this error, write digits after `0b`:
+
+    let mask = 0b1000;
+
+Alternatively, remove `b` to create a `0` number literal.

docs/errors/E050.md

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+# E050: hex number literal has no digits
+
+Hexadecimal (hex) number literals start with `0x` and require at least one
+digit. It is an error to write `0x` with no following digits:
+
+    let mask = 0x
+
+To fix this error, write digits after `0x`:
+
+    let mask = 0xff00;
+
+Alternatively, remove `x` to create a `0` number literal.

docs/errors/E051.md

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+# E051: octal number literal has no digits
+
+Octal number literals start with `0o` and require at least one digit. It is an
+error to write `0o` with no following digits:
+
+    let mask = 0o
+
+To fix this error, write digits after `0o`:
+
+    let mask = 0o700;
+
+Alternatively, remove `o` to create a `0` number literal.